1. Field of the Invention
This invention relates to electromagnetic shielding for copper and optical interfaces with pluggable transceivers and in particular to shielding used for parallel interfaces such as used on InfiniBand optical links. More specifically, this invention relates to pluggable transceivers for both copper and optical parallel interfaces.
2. Description of the Related Art
Pluggable interconnects are an industry trend and have already been accepted in other standards. Specifically, serial transceivers with pluggable interfaces are available today at data rates up to 10 gigabits/second (Gbit/s), and parallel interfaces with similar per line data rates are available. There are a number of reasons why pluggable interfaces have been accepted by the industry. First, they provide the ability to easily switch between different types of physical layers, such as between copper and optical or between short-wavelength and long-wavelength optical links for increased distance. This can be done either in the field, with the transceiver as a replaceable unit to reduce field service costs, or at end of the link manufacturing process to reduce the cost of customizing a card and minimize the use of unique card part numbers. Since a transceiver is typically the only pin-through-hole (PTH) solderable component on a card, migration to a pluggable interface with a surface-mounted receptacle can result in significant manufacturing cost reductions (e.g., 20-30%). Furthermore, the industry is moving to new environmental regulations that require lead-free solders and higher soldering temperatures. By using pluggable transceivers, one can avoid exposing the transceiver to elevated temperatures during card build or rework. This significantly improves reliability, especially for optical links whose lasers are sensitive to temperature. Since the optical transceiver is often one of the most complex and failure-prone components on a card, this is an important benefit.
However, the combination of pluggable transceivers with higher data rates and parallel links leads to a new set of design problems. In particular, there is a concern with the resistance to electrostatic discharge (ESD) and electromagnetic interference (EMI) leaking onto and off the cards over these interfaces. Consider the example of a parallel optical link, with 12 lasers having a 250-micron (0.25-millimeter) center-to-center spacing in a single package. Each laser is driven by differential serial lines at 10 Gbit/s. Because there are so many high-speed serial lines close together, their radiated noise adds up and can produce higher field strengths within the module than a single 10-Gbit/s serial device. This high frequency noise can escape from the transceiver or card package through apertures as small as 0.07 millimeter (mm). Although high frequencies attenuate quickly, applications such as high-density switches may place tens of transceivers in closely packed proximity to one another on a card. This noise can also re-radiate from secondary sources, such as metal in the cable connectors. Thus, even small noise leakage can cause the switch to violate FCC class B radiated noise requirements unless the aperture around the transceiver is properly shielded. High-frequency noise from adjacent switch ports can also be picked up without proper shielding and introduce bit errors on the link. Since the transceivers are mounted on the card edge with direct access to the outside environment and serve as a user interface for plugging cables, they are further exposed to ESD shocks from handling of cables, connectors, and cards.
Thus, proper EMI/ESD shielding of these new interfaces is critical. However, conventional shielding approaches may not be adequate because of the pluggable transceiver design. As shown in FIG. 1, the dimensional tolerances between the edges x-x and y-y of a transceiver module (not shown) and the corresponding edges 102 and 104 of a typical PCI-type adapter card 100 are ±20 mils (≈0.5 mm). Thus, a pluggable transceiver receptacle can be placed on the card within 20 mils of a card edge. However, most industry optical component multi-source agreements specify that the length of a parallel transceiver has a tolerance of ±1 mil (≈0.025 mm). This leaves a potential gap between the transceiver nose and the card edge, which needs to be shielded. In the past, transceivers were soldered onto the card and the nose was grounded to the card bezel by flexible metal fingers on an EMI shroud or a gasket which would compress up to 50% when the card bezel is assembled. This approach is not workable for a pluggable transceiver; further, it is probably not adequate to shield against higher-frequency noise on the new parts. Since the transceivers can be inserted or removed in the field, a fixed compression gasket or shield on the transceiver nose is not an option, either. Therefore, an object of this invention is to develop an acceptable way to shield the transceiver opening on a pluggable, high-data-rate parallel interface.
There have been a number of previous efforts to shield or control electromagnetic radiation from various types of apertures. Each of these previous efforts, however, has its drawbacks.
Byrum et al., in their publication “Pluggable EMC Shield and Cable Strain Relief System”, IBM Technical Disclosure Bulletin, vol. 30, no. 6, pp. 76-78 (November 1987), describe a metal cable collar and conductive rubber cable-receiving rack designed to provide both strain relief and electromagnetic conduction noise (EMC) shielding of copper cables. The teachings of this publication cannot be usefully applied to fiber optic ribbon cables used for InfiniBand, which have a significantly smaller diameter than the copper cables assumed by this reference. (InfiniBand is a trademark of the InfiniBand Trade Association.) This publication describes a method for clamping and screwing the shield around the cable, which would damage a fiber optic cable or induce microbending loss. Further, this shield is applied to the cable itself, not to the card edge connector. Since all of the noise from a fiber optic interface is conducted through the card edge (the glass and polymer fiber cable is nonconductive), the disclosed system would not be useful for fiber optic cables.
U.S. Pat. No. 5,960,136 (Shakhman et al.) describes a conductive shield formed around a fiber optic connector and panel opening, using conductive metal fingers or springs which extend from the optical connector and contact the card edge receptacle (which is also assumed to be conductive). These fingers or springs are intended to compress against the card receptacle when the fiber connector is inserted. The patent describes an array of fingers or springs surrounding the fiber optic connector, which is only practical for relatively large connectors. This approach would not scale well to an MPO-type ribbon connector used in parallel optics. Further, this approach still leaves gaps between the fingers or springs which are susceptible to radiated noise leaking into or out of the optical interface.
U.S. Pat. No. 6,158,899 (Arp et al.) describes a two-piece rigid metal or metallized plastic assembly which can be clipped over a fiber optic connector and receptacle after the cable has been plugged in. The two-piece shield makes contact with the card receptacle and forms a shield around the optical connector. This approach requires that the shield be assembled around the fiber optic interface after the cable has been plugged in; the shield must be removed in order to unplug the cable. This has several drawbacks; it requires a separate mechanical feature on the receptacle, such as a slot or screw hole, to attach the two-piece shield and make electrical contact with the card edge. It is cumbersome to attach and remove the shield every time the cable is plugged. In the case of InfiniBand, the transceivers themselves may be pluggable into the card, which further complicates the process (in order to unplug a transceiver, the shield must be removed, the cable unplugged, and then the optics removed, while the process is reversed to re-insert the optics). The shield is disassembled into two parts, so one or both may be misplaced during the assembly process. A molded metal shield will be heavy (possibly bending the optical fibers) and likely expensive; a metallized plastic shield is susceptible to scratching, which would compromise the protection.
U.S. Pat. No. 6,241,398 (Correa et al.) describes a conductive tubular member (such as an ST-type single-fiber connector), made of an electrically conductive elastomer, which slides over a nut projecting beyond a faceplate to form an EMC shield when the optical connector is inserted into the receptacle. This takes advantage of the fact that ST-type connectors require a metal receptacle with locking pins formed in the proper locations to fully engage the optical connector. This approach does not extend to an MPO-type ribbon connector, which does not have a metal sleeve and may not even be conductive. While this approach is useful for single fiber connectors with metal receptacles like the ST, it does not apply to nonconductive plastic connectors for ribbon fibers.
U.S. Pat. No. 6,482,017 (Van Doom) describes an EMI shield, strain relief boot, and dust cover for optical interfaces. The design goal is protection against electromagnetic interference (EMI), and does not explicitly include EMC protection. The required size of this approach is larger than it would otherwise have to be, since it also needs to serve as a strain relief and dust protection for the interface. The EMC shield incorporated into the boot consists of either a plurality of conductive wires, a plurality of conductive particles, a magnetic material, or a conductive layer incorporated into the boot structure. This extends over most (but not all) of the area surrounding the optical receptacle; some of the area is left unprotected. The boot tapers towards the receptacle, and does not insure physical contact between the boot and the card edge; there is also no guarantee of electrical contact between the boot and a conductive surface on the card, to protect against EMC (the boot only contacts the optical connector receptacle, which may be nonconductive plastic; the invention could be modified to include metal edges on the receptacle, but this would require an additional feature). The resulting gaps or spaces may reduce the shielding effect. Finally, the boot body must be configured to limit the cable bend radius.
PCT Patent Publication WO 02/079841 (Robinson et al.) describes a conductive cable boot and shield based on the waveguide-beyond-cutoff principle; the dimensions of the boot are such that it will not support propagation of EMI noise modes beyond a predetermined cutoff frequency, thus preventing the noise from coupling out of the receptacle. The boot is constructed such that its typical length is at least five times its width, thereby blocking EMI with destructive interference. This is much larger than would ordinarily be required for a shield; also, the dimensions are relatively critical. (For example, the patent publication describes this boot as having an internal angle of 18.5° with a tight tolerance in order to provide the waveguide cutoff function.) Further, InfiniBand uses arrays of lasers and drive circuits in close proximity, and may generate harmonics that penetrate a shield based on waveguide principles.
U.S. Pat. No. 6,568,861 (Benner et al.) and U.S. Pat. No. 6,607,303 (Ngo et al.) are very similar in that both describe a fiber optic adapter mounted through a panel opening which makes conductive contact with the opening in order to shield cables which pass through the aperture. This is basically a split sleeve LC-type optical coupler with metal shielding to protect the optical interface between two fiber connectors. This approach does not apply to the interface between an optical connector and a card edge receptacle, since by its nature the coupler extends into both sides of the aperture; thus it cannot accommodate an optical transceiver, which would have to extend partially through the aperture and which does not have a conductive receptacle.